Ford only use 2 spline counts in the light duty trucks, 28 and 31. Yours most likely is 28, so that is what you will need to find.

I changed my truck's rear gears from a 3.50 to a 2.75 - but I also went to a much more powerful engine, a 460. The 460 can produce the extra torque needed to get the heavily loaded truck moving from a stop with those gears, whereas a 300 or 352 engine cannot.

As for your '71, keep in mind your basic physics. If you want to get speed out of your '71, you will lose your torque. That is probably something you do not want to do if that is your towing and working vehicle.
Even if you do try to make it work, the clutch will burn out much faster due to the extra slipping it has to do to get going.